Raster type printers, which have been implemented with various print engines commonly found in the arts, such as electro-photographic print engines and ink jet print engines, employ half-toning to transform continuous tone image data to print data that can be printed as an array of dots that can be of substantially similar size. For example, 24 bit/pixel continuous tone image data can be half-toned to a plurality of single color one-bit per pixel bitmaps.
Half-toning may employ a screen having a matrix of different threshold values. A screen can be a data set with different print density values equally represented (or with a controlled unequal distribution for gamma-compensated screens). For monochrome printing, the image data is then compared with the screen thresholds at each position. If the image data exceeds the threshold, a dot is printed. Otherwise, that particular location remains unprinted.
Improved appearance can be provided using, for example, a pseudo-random stochastic screen having a “blue noise” characteristic. Such screens tend to have threshold values which are distributed so that adjacent values tend to be very different. Thus, any value or limited range of values will tend to be located at positions that are nicely spaced apart on the matrix. In this example, apparently even but random spacing can be emphasized at very low and high density values in a blue noise screen.
For printing with multiple colors, half-toning presents a particular challenge. For dot-on-dot printing, in which printed locations are printed with one or more dots, a single half-toning screen can be used. For instance, a field of 10% blue would have 10% of locations printed with cyan and magenta ink, while 90% of locations remain unprinted. This has the disadvantage of reduced spatial frequency with respect to methods that distribute dots to different locations. This also tends to give the appearance of darker dots more widely spaced apart, producing a grainy image. The same can be said for clustered dot printing techniques in which different color dots may be printed adjacent to each other or otherwise clustered to create a multi-dot cluster that reads as an intermediate color. Accordingly, it is desirable to print the individual dots at closely spaced separate (non-overlapping) locations, relying on the viewer's eye to integrate the different color dots into the intended color.
By using different screens having the threshold values arranged differently, the dots will tend not to align with each other. However, with uncorrelated screens, the printed patterns of different colors will tend to be randomly located with respect to each other. This can generate some graininess of an image as some dots happen to clump near others or overlap. To reduce this with two colors, an inverted screen can be used for one of the colors. An inverted screen often has values equal to the maximum screen value (less the screen value at the corresponding location on the other screen). Thus, 10% blue is printed by printing cyan dots at all locations where the threshold values of the original screen are 25 or less. Magenta dots are printed at locations of values of 230 and above on the original screen (25 or less on an inverted screen). Inverted screens can be limited in usefulness for several reasons.
First, inverted screens may only be used for two colors. This can be inadequate for most multiple color printing systems. Where image quality is not critical in tri-color Cyan, Magenta, Yellow (CMY) systems, the darker C and M dots may be printed in this way while the less visible yellow dots may be distributed otherwise. For four-color systems employing black ink and for multi-level grayscale printing, the inverted screen may not provide desired image quality.
Second, for two color systems where one color is printed at the lowest value range positions, and another is printed at the highest value range positions, those positions are not relatively well dispersed with respect to each other in a blue noise screen. Although it will not generate overlapping droplets at less than full coverage printing, a high frequency blue noise screen may lead to clumps of adjacent dots. Beyond the random effects leading to such clumping, widely different values are more likely to be adjacent to each other.
For three and four color systems, a shifted screen approach has been employed to avoid pure dot-on-dot printing for some colors. This can lead to increased graininess of the image but often generates unwanted low frequency artifacts that are visible in the printed image. Moiré patterns may also be generated. It can be difficult to achieve substantial uniformity or even distribution of the half-toned dots in dot-on-dot printing devices.
Accordingly, what is needed in this art are increasingly sophisticated systems and methods for minimizing dot visibility by reducing halftone dot graininess in output color images printed by ink jet products capable of dot-on-dot printing.